Science Buddies: "Ask an Expert"

My son, a fifth grader, has a science fair project about Batteries and he needs to measure different brands of batteries to see which one lasts longest. So I was thinking of having him build his own flashlight using the guide from Energizer found at http://www.energizer.com/learning-cente ... light.aspx. We will connect this flashlight to 3 or 4 different brands of batteries and have the flashlight on until the batteries run out.

So I have a few questions for the Science Buddies community:

1. What do you think of the Energizer flashlight, is this a good way of building the flashlight or is there a better approach? Can the materials be replaced with something better (e.g. instead of paperclip use some kind of switch) to make it more presentation-friendly?

2. What is the best way for us to measure the battery life? I was thinking it would be nice to connect some sort of stopwatch to it that would turn off when the batteries die and record the duration but I am not sure if that is even possible. Do I need to use an ammeter? If so, how do you measure battery life, with this device, assuming we leave the flashlight on overnight?

3. Do you have any recommendations of other projects ideas around battery life measurement appropriate for a 5th grader?

Thanks for your help.
- Elmer

The link you cite uses a PR6 bulb and two D cells to build a flash light. This isn't the most efficient way to test battery life for several reasons:
1) The amount of light produced and electricity used will be a decreasing function of how much voltage and/or current two batteries in series are able to produce which will diminish with time. When do you call it exhausted? Any variation in your choice of what you call exhausted might affect your results.
2) You are testing a series of two batteries either of which maybe the weakest of the two. It would be better to use a single cell circuit so that each test result is based on a single battery sample.
3) Some PR6 bulbs have a published expected life of around 30 hours which means your test load can also contribute to an early failure indication.
4) You really don't want to have to watch the flashlight to determine the failure point.
See page 2 of http://data.energizer.com/PDFs/E95.pdf for an example of some graphs that show constant resistance and constant current expectations for an example of what a good data sheet tells you for one particular battery. If you assume that a PR6 draws an average of about 300 mA, you are up in the 40-50 hour range.

How much any given fifth grader is able to learn about all the aspects of electricity and various electrical loads is largely a function of how inquisitive and motivated they are. This isn't about YOU doing it, it is about your son doing it. Is your son motivated enough to research and learn about ohms law and the related power equations? If so, then he can probably come up with a load resistor circuit and use a digital volt meter that can be used to simulate a load and run a test. If you are willing to invest in building some single cell battery holders and buying multiple load resistors and multiple "switches", you can run concurrent tests with several different batteries on some weekends where you can run them for several hours taking measurements every hour and then turn them off when you get tired and start in again the next day. Same can be done in the evenings.

Beware: You need to use the same pattern of hours on and hours off on all test samples to be fair as there is some ability of some battery types to "recover" somewhat in the "off" periods. You need to test at least three samples of each type of battery. Batteries have shelf lifes so you need to try and get samples that have not been sitting on shelves or in a warehouse for long periods of time.